Swift - Parse a String to extract and edit numbers in it [closed] - swift

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Given a String taken from the internet, such as:
"A dusting of snow giving way to moderate rain (total 10mm) heaviest on Thu night. Freeze-thaw conditions (max 8°C on Fri morning, min -2°C on Wed night). Mainly strong winds."
Using Swift 3, I want to convert the temperatures to Fahrenheit. So I need to find any numbers that have °C after them (including negative numbers); convert them to Fahrenheit, and then replace the integer back into the string.
I was originally trying to use: components(separatedBy: String). I did get it to work with this method. Although I think there is probably a better way.
func convertStringToFahrenheit (_ message: String) -> String{
var stringBuilder = String()
let stringArray = message.components(separatedBy: "°C")
for subString in stringArray {
if subString != stringArray.last {
if subString.contains("(max "){
let subStringArray = subString.components(separatedBy: "(max ")
stringBuilder.append(subStringArray[0])
stringBuilder.append("(max ")
if var tempInt = Int(subStringArray[1]){
tempInt = convertCelsiusToFahrenheit(tempInt)
stringBuilder.append(String(tempInt))
stringBuilder.append("°F")
}
}
else if subString.contains(", min "){
let subStringArray = subString.components(separatedBy: ", min ")
stringBuilder.append(subStringArray[0])
stringBuilder.append(", min ")
if var tempInt = Int(subStringArray[1]){
tempInt = convertCelsiusToFahrenheit(tempInt)
stringBuilder.append(String(tempInt))
stringBuilder.append("°F")
}
}
}
else {
stringBuilder.append(subString)
}
}
return stringBuilder
}

Answer 1

A job for regular expression.
The pattern "(-?\\d+)°C" searches for
an optional minus sign -?
followed by one or more digits \\d+
followed by °C
The group – within the parentheses – captures the degrees value.
var string = "A dusting of snow giving way to moderate rain (total 10mm) heaviest on Thu night. Freeze-thaw conditions (max 8°C on Fri morning, min -2°C on Wed night). Mainly strong winds."
let pattern = "(-?\\d+)°C"
do {
let regex = try NSRegularExpression(pattern: pattern)
let matches = regex.matches(in: string, range: NSRange(location: 0, length: string.utf16.count))
for match in matches.reversed() { // reversed() is crucial to preserve the indexes.
let celsius = (string as NSString).substring(with: match.rangeAt(1))
let fahrenheit = Double(celsius)! * 1.8 + 32
let range = match.range
let start = string.index(string.startIndex, offsetBy: range.location)
let end = string.index(start, offsetBy: range.length)
string = string.replacingCharacters(in: start..<end, with: String(format: "%.0f°F", fahrenheit))
}
} catch {
print("Regex Error:", error)
}
print(string)
The most complicated part of the code is the conversion NSRange -> Range<String.Index>